Alarm devices for preventing an intruder from entering a window or other aperture have had a long and full design history. They have included networks of electrical sensor wires disposed across window openings, protective screening with electrical circuitry included therein, optical fibers embedded in panes of glass or made into grills of fiber-optic rods and wires embedded in glass panes.
However grills of electrically conductive frangible rods, i.e. thin brittle rods which readily snap and generate an alarm if an intruder tries to dislodge or force his way by them, as hereafter described in reference to the present invention, are believed to be unknown in the prior art. Hollow rods or pipes with internal electrical conductors have been employed for burglar alarms as in U.S. Pat. No. 2,374,139 but they have been of metal to function as window bars as well as conductive alarms. In contrast to sturdy bars of that sort, easily broken or melted alarm-generating conductive strips of tinfoil or the like have been used but they have been applied directly to window panes as in U.S. Pat. No. 1,578,980. Frangible conductive elements for purposes other than burglar alarms are known, as for example the brittle electrical sensors of U.S. Pat. No. 3,936,621.
Perhaps the closest prior art to the invention is U.S. Pat. No. 3,725,891. While it discloses a grid of breakable plastic tubes containing a conductor, the tubes are mounted in a rigid frame and subframes, all of which must be sized to a particular window opening. The grid of tubes of this reference is therefor not free standing and cantilevered across the window opening, and indeed is a kind of open-mesh screen similar to protective alarm screens such as that of U.S. Pat. No. 3,051,935. In addition, the conductor within the tubes is an insulated wire which would require a rather considerable breaking force.